The present invention relates to a full duplex data transmission system using an echo canceller, and more particularly to an echo canceller for a full duplex data transmission system suitable for using a 2B1Q code as the transmission code.
Various transmission codes have so far been proposed for application to full duplex data transmission using an echo canceller. Among the proposed codes, a 2B1Q code by which a two-bit code is converted into four signal levels (+3, +1, -1, -3) have been adopted as the standard code in the U.S. because of its superiority over other codes including an alternate mark inversion code and a biphase code in terms of the required transmission band and the effect on near-end crosstalk. While no specific circuitry has as yet been proposed for an echo canceller system suitable for a 2B1Q code, there is conceivable in general a configuration using a well-known echo canceller and a digital-to-analog (D/A) converter for converting the two-bit code into the four-level signals. The use of a D/A converter, however, involves the problem of complicating the circuitry.